This is a basic stacked-tube-blowback (STBB) to pump conversion tutorial. This conversion should work with any STBB including Spyders, Brass Eagles, Piranhas, ZAPs and Draguns. Although I haven't done this mod, I've read enough tutorials (listed as references) to figure out what makes the marker work. When finished, your marker should go from this:
to this:

Parts:Completely round valve pin. The easiest to use are Dragun “The One” valve pins and Trilogy Sport Autococker valve pins, but are hard to find. Wulf created one out of a threaded rod from a rifle cleaning kit. Vikingshadow modded the stock valve pin by roughing up the flat areas, covering them with JBWeld, sanding the JBWeld down to match the pin itself, and then lightly polishing the modded pin to allow for smooth travel in the valve body.
Courtesy of Fubarius:
Fully round valve pin on the left, stock valve pin on the right.

Slotted striker. This will have to be cut with a Dremel/rotary tool or milling machine. The slot needs to be long enough to allow for the bolt to slide into a fully uncocked position while the striker is still cocked.
Courtesy of Fubarius:
Slotted striker on top, stock striker on bottom. Note: your slot does not necessarily have to go past the o-ring.

Basic functions:
This section is to explain what each modification does to make the pump marker functional.

Modifying the valve pin. A standard STBB valve pin allows air to flow not only into the bolt, but also allows the air to push the striker back and recock it. Using a completely round valve pin will eliminate the flow of air into the lower tube (blowback), thus keeping the striker from automatically recocking.

Modifying the striker. Cutting a slot into the striker allows the striker and bolt to move independently. First, the bolt will be cocked and uncocked to load the ball into the chamber. This will also cock the striker. Then the pull of the trigger will release the striker even when the bolt is in the forward (uncocked) position. The striker will open the valve (via opening the gap between the valve body and cup seal) and allow the air to flow up through the hole in the top of the valve and into the hole in the bottom of the bolt, thus projecting the paintball out of the barrel.

The pump handle. The pump handle simply allows you to manually recock the bolt without taking your hand off the marker.

Putting it all together:
First, you need to take your marker apart enough to take out the valve pin and striker. Replace the stock valve pin with the completely round one. Then replace the stock striker with the modded one, if you didn't mod the striker that originally came with the marker. After you have reassembled the marker, attach the pump handle and connect the pump arm(s) to the bolt.

There are many ways to create a pump handle. Some people use PVC pipes, cans, or whatever else they have lying around. The cleanest way though is the Trilogy Autococker Pump Conversion Kit from PMP Industries. The pump handle threads into the low pressure chamber (LPC) or front plug opening of most STBBs. Vikingshadow's Spyder Sonix and Fubarius' VL Charger use this method. For home made pump handles, you have two options: under-barrel cocking or barrel cocking. The first method involves the pump handle being located under the barrel, while the second involves some kind of pipe being slipped over the barrel like a sleeve. bequickorbedead uses a combination of the two by attaching rings that slide over the barrel to the top of the pump handle. In his bolt action rifle mod, RTisgood uses the bolt from an actual rifle to move the Spyder bolt and cock and load his marker.

How you attach the pump handle and arm to the bolt is up to you. Some people put a metal plate behind the bolt and connect the pump arms to the plate while others connect the pump arm directly to the bolt with JBWeld or by tapping a threaded hole in the bolt, threading the end of the pump arm, and screwing the arm into the bolt. Here are a few different methods:
~Metal plate:WulfCodymm1014
~Bolted to boltFubarius
~Glued to boltbequickorbedead

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I have a better way that doesnt require reworking the hammer or moding/swapping valves. kinda sorta in a way operates like an MQ pump. Allyou do is
remove the bolt pin,
glue it in the hammer
cut the pin down so the bolt and hammer can move independently of eachother
add a pump arm to the bolt that will prevent the bolt from spinning around
add some sort of return spring to the pump

To use it you assemble it so the hammer is cocked back, gas it up, chrono and go play.

I have a better way that doesnt require reworking the hammer or moding/swapping valves. kinda sorta in a way operates like an MQ pump. Allyou do is
remove the bolt pin,
glue it in the hammer
cut the pin down so the bolt and hammer can move independently of eachother
add a pump arm to the bolt that will prevent the bolt from spinning around
add some sort of return spring to the pump

To use it you assemble it so the hammer is cocked back, gas it up, chrono and go play.

That will indeed get the job done, but won't be a "true" pump in the technical sense, since the pump action only serves to reload ball into the breech, and not to recock the hammer.

haha, thanks. this is my contribution to society, so don't expect anything else

vike, i'm working on a rewrite right now, so that will be up in a bit.

the thing that really surprised me was that i did this in almost exactly 2 hours. i had almost everything bookmarked/subscribed (plus a few links the guys in KOT threw out ) so i had all the resources right in front of me.